Why wildfires are raging so close to U.S. cities this year

A confluence of factors has sparked one of the U.S.'s most active wildfire seasons in recent memory, upheaving thousands of residents, and generating so much smoke it's blanketing the Midwest. Not only that, the fires are uncomfortably close to several big cities, making it even more difficult for emergency crews to address them. Why is the country suddenly aflame?

The National Interagency Fire Center reported yesterday that about 7.8 million acres have burned in the U.S. this year. "While it is unlikely that this season will be record-breaking for modern fire record keeping in the western United States, it is above normal relative to the last decade—which has seen abundant fire activity," said John Abatzoglou, a climate and atmospheric scientist at the University of Idaho.

So what exactly is making this year's fire season so devastating—and so widespread?

Extreme heat got us into this mess

Let's start with the obvious: It's been hot. All-time temperature records have been broken up and down the West Coast over the last few weeks. San Francisco hit 106 last Friday (the old record was 103 degrees) and a slew of Bay Area cities also topped their previous highs.

It's not just heat, however, it's the combination of heat and low humidity sticking around. The West has been gripped by a high-pressure system that's prevented the current hot, dry weather pattern from moving along. Amazingly, this same high-pressure system locked Hurricane Harvey into a crawl over Houston, breaking the continental U.S.'s all-time rainfall total for a single storm.

One interesting sidenote: In some places, the heavy rains that busted the drought may have actually made the fire season worse. So much vegetation has grown and now dried out that the fires have more fuel to burn than they did during the drought summers.

Smoke is in many ways just as dangerous

Even if you're hundreds of miles from an active fire, there's a good chance you're affected by the smoke—which is spreading evidence of the fires and their destruction in a very visible (and very unhealthy) way. The jet stream is currently positioned over the biggest fires in the Pacific Northwest, dragging that smoke .

Sprawl is pushing homes into dangerous territory

Wildfire isn't a new threat—it's been destroying cities for centuries. But the way we've been building cities over the last few decades is making those fires far more destructive. More Americans are moving into what's known as the wildland-urban interface (WUI), where suburbia butts up against wilderness areas. Since the late 1970s, the number of Americans living in WUI lands has doubled, and 60 percent of new U.S. homes have been built on WUI lands since 1990.

As my colleague Patrick Sisson wrote last week, that kind of growth is creating a big nightmare for insurance companies that are trying to manage fire risk. Not only are more structures threatened by fires, it's also costing cities (and state and federal governments) more to evacuate residents. The U.S. Forest Service now spends about half of its budget on firefighting, compared to only 13 percent in 1995.

Forests are overdue for burns

Many forests are supposed to burn as part of the natural ecological process—it not only replenishes the soil, but also helps to clear fallen timber that will prevent even bigger fires. Which is why some forest management plans have turned to prescriptive burns which set small, controlled fires that will help mitigate risk down the road.

The problem with this approach, of course, is not only that cities have allowed people to live too close to these now-overgrown forests, making prescriptive burns impossible, but also that people living nearby don't want their open space torched—even as a precautionary move in the name of safety.

Climate change lengthens the fire season

While we can't directly attribute the fires to a warming planet—there's a whole range of variables—there is consensus among scientists that the increase in greenhouse gas emissions in the atmosphere which are known to warm the planet have extended the fire season, perhaps by as much as 20 percent since 1979. So there's a much larger window for these types of incidents to occur.

While the lengthened season certainly makes , it's even more frightening for regions that aren't used to experiencing wildfire at all. Fires are now starting in parts of Alaska and Siberia that have never burned before. In fact, 2017 has already seen a rare wildfire in Greenland.